Got the transmission up on a table and started de-griming it.
I’m still thinking about how best to go about this. I can see some dust and metal shavings inside the transmission… I’m thinking of a flush with regular motor oil to get it mostly cleaned out. I guess the decision comes down to whether I should drive it around on the motor oil (higher rpms) or seal and fill it as is and turn the input shaft with a drill..
I decided against pulling the transmission apart to inspect it. Instead I used a screwdriver on the shift linkage to step through most of the gears. It seems really crisp, and all of the gears I tried (1,2,5,R) seemed to mesh and turn without any grinding or anything. Hopefully everything’s good internally.